1. Field of the Invention
This invention relates to a catalyst for the purification of exhaust gas capable of purifying exhaust gases from internal combustion engines, particularly, from atuomotive engines and of removing carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxides (NOx) existed in the exhaust gases.
2. Prior Art
A conventional catalyst for purifying automobile exhaust gases usually comprises a catalyst carrier layer and catalyst ingredients carried thereon and, so far, there have been demonstrated various kinds of catalyst aiming at improving the purification property. For example, some catalysts using cerium (Ce) have been disclosed in Japanese Published Examined Patent Application Nos. 41775/1984 and 20307/1983, and Japanese Published Unexamined Patent Application No. 90695/1984. In these catalysts using cerium, the cerium is present in the oxide form. The cerium improves the purification property by discharging or storing oxygen (oxygen storing capability) according to the reaction shown in the formula (1) and by controlling the oxidation reaction of CO and HC and the deoxidation reaction of NOx. ##EQU1##
It has been known that the reaction shown in the formula (1) appears on the particle surface of cerium oxide. However, in the aforesaid conventional catalysts for purifying exhaust gases, the total surface area of the cerium oxide particles was reduced by the growth of the cerium oxide particles on using the conventional catalysts at elavated temperatures over 800.degree. C. Accordingly, there has been arised a disadvantage that the purification property is degraded due to the worsening of the oxygen storing capability.
Also, some catalysts using rare earths and transition metals simultaneously with cerium for aiming at the stabilization of active alumina, were disclosed in Japanese Published Examined Patent Application No. 7537/1985, Japanese Published Unexamined Patent Application Nos. 18180/1973 and 3531/1986, U.S. Pat. Nos. 3951860 and 4170573. For example, a catalyst disclosed in Japanese Published Examined Patent Application No. 7537/1985 has used cerium with lanthanum (La) and formed a composite oxide shown in the formula (2). ##EQU2##
In this catalyst for purifying exhaust gases of the formula (2), a lattice defect with oxygen vacancies is formed in the fluorite structure of the composite oxide, so that the oxygen storing capability with durability can be achieved. However, even in this catalyst for purifying exhaust gases of the formula (2), the growth of particles, which is not so much as that of the formula (1), is increased in the cerium oxide and the purification property is worsened. In this catalyst for purifying exhaust gases of the formula (2), the volume of lanthanum is contracted since LaAlO.sub.3 was formed at elevated temperatures. Therefore, internal stress is produced in the alumina catalyst carrier layer and depositiong strength is degraded between the alumina catalyst carrier layer and the support substrate. Thus degraded depositing strength causes another disadvantage that the alumina catalyst carrier layer is peeled off.
Accordingly, it is an object of this invention to provide a catalyst for the purification of exhaust gases capable of ensuring the excellent purification property by suppressing the growth of cerium oxide particles at elevated temperatures, and also capable of preventing the formation of LaAlO.sub.3.